Hollow Fiber Supported Liquid Membrane for the Separation

The application of a hollow fiber supported liquid membrane (HFSLM) for the simultaneous recovery and concentration of gold(I) from alkaline cyanide m...
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Ind. Eng. Chem. Res. 2000, 39, 146-154

Hollow Fiber Supported Liquid Membrane for the Separation/ Concentration of Gold(I) from Aqueous Cyanide Media: Modeling and Mass Transfer Evaluation Anil Kumar† and A. M. Sastre* Departament d’Enginyeria Quı´mica, Universitat Polite` cnica de Catalunya, ETSEIB, Av. Diagonal 647, E-8028 Barcelona, Spain

The application of a hollow fiber supported liquid membrane (HFSLM) for the simultaneous recovery and concentration of gold(I) from alkaline cyanide media using microporous hydrophobic polypropylene hollow fiber is considered. In HFSLM configurations, the organic extractant used for gold(I) extraction was 2-16% (v/v) LIX 79 (Hankel Corp.) diluted with n-heptane. The study of HFSLM includes the influence of hydrodynamic and chemical conditions, i.e., the flow rate of feed solution through the fibers, the lifetime of the system, the initial Au(I) concentration in the feed solution, the NaOH concentration in the stripping phase, and the pH of aqueous phases. A model is presented which describes the transport mechanism, indicating different ratecontrolling steps. The validity of this model was evaluated with experimental data and found to tie in well with theoretical values. It was possible to achieve a 35-40-fold concentration in product (NaOH) in the presence of other metal cyanide salts such as Fe(II), Cu(I), Ni(II), and Ag(I) using the HFSLM technique. The effect of excess cyanide in the feed up to 1000 ppm did not affect the recovery of gold. Introduction Today, the world’s gold is produced by hydrometallurgical techniques including combinations of leaching, adsorption, and electrowinning or precipitation steps.1 The gold is extracted from the ore using a basic cyanide solution with a leaching reaction described as

4Au(s) + 2H2O + O2(g) + 8CN- S 4Au(CN)2- + 4OH- (1) When dealing with mineral ores or industrial wastes containing precious metal such as gold, one of the most effective leaching alternatives is the use of cyanidation. The gold mineral processing industry therefore uses cyanidation steps. In this case, cyanidation leaching steps are combined with subsequent steps of goldcyanide adsorption with activated carbon. This technique has certain apparent drawbacks such as lack of selectivity due to the presence of mixtures of other metal cyanides when adsorbed on activated charcoal and its complexity through involving a multistep scheme for separation. For this reason, in the case of the gold industry, the needs are directed mainly toward development of other new techniques that could efficiently substitute the use of activated carbon.2,3 In view of this, there is an absolute necessity to develop a simple separation scheme with minimal steps. Recent developments in membrane technology include one promising idea involving the use of a hollow fiber membrane based process such as hollow fiber supported liquid membranes (HFSLMs) which affords high mass-transfer rates of solutes, especially with high selectivity by the * Corresponding author. Fax: (34)-93-4016600. Telephone: (34)-93-4015823. E-mail: [email protected]. † On leave from PREFRE, Bhabha Atomic Research Centre, Tarapur, India. Fax: +91-252-572866.

use of extractant.4 The capacities of performing selective metal extraction and treating dilute solutions make the HFSLM technique an attractive alternative to solvent extraction in that it combines the processes of extraction stripping and regeneration in a single step (HFSLM). As compared with solvent extraction, HFSLMs are characterized by being rapid in separation, high in efficiency, low in power consumption, and adaptable to diverse uses. In recent studies, the results for extraction of Cd(II), Ni(II), and Zn(II) with bis(2-ethylhexyl) phosphate/isododecane using hollow fiber contactors were compared with those for a pulsed sieve-plate column and ideal mixer settler cascade. It is noted that one module (50 cm long, 9000 fibers, and area of 1.1 cm2 or 25 cm long and 10 000 fibers, and area of 0.5 m2) can replace the extraction column of 6 m in length and two to four ideal stages.5 Moreover, operational problems generally encountered in liquid-liquid extraction such as flooding and loading limits in continuous countercurrent devices, the need for a density difference between the phases, third phase formation, emulsion formation, and phase entrainment could be eliminated in HFSLM owing to nondispersive contact at the pore mouth of hollow fiber membrane.8 Furthermore, HFSLM techniques have been extensively deployed in separation science such as, for example, metal recovery from leaching and wastewaters, winning of precious and strategic metals from neutral waters, and treatment of large volumes of effluents including toxic and hazardous wastes generated by industries.6-10 Interestingly, from cyanide media, several amine extractants have been used for extraction of Au(I) at pH > 9, but their performance was reported to be poor because they did not fit the process requirement, mainly due to the low basicity of amines. They thus proposed modified amines (increased basicity) to extract Au(CN)2- from high pH from alkaline cyanide media.11 To overcome this difficulty, a single organic reagent is therefore desired,

10.1021/ie990267a CCC: $19.00 © 2000 American Chemical Society Published on Web 12/03/1999

Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 147

which should have good extractability at pH > 9 and stripping of Au(I) in highly basic (pH > 12) solutions. Available single extractants tested do not meet the above objectives in the pH cycle, with the exception of the recently developed family of reagents from Henkel Co. containing a guanidine functionality.12,13 A new chemical being marketed by Henkel, namely, LIX79, was therefore chosen for Au(I) extraction which overcomes the aforementioned difficulties.14 In the case of HFSLMs, the calculation of the overall permeability coefficients of the experimental system is based on a first-order mass transfer model with instantaneous chemical reaction on the stripping side, when a recycling mode is employed. The study of the influence of the liquid membrane composition leads to the evaluation of the mass-transfer parameters. Experimental Procedure Reagents. A stock solution of Au(I) (5 g/L) was prepared from pure solid KAu(CN)2, and 1 g/L of each cyanide salt such as KAg(CN)2, Zn(CN)2, KNi(CN)4, and CuCN (Johnson Mattey Chemicals, Karlsruhe, Germany) and K4Fe(CN)6‚3H2O and NaCN (Merck, Darmstadt, Germany) were dissolved in NaCN (Merck). KNi(CN)4 and CuCN salts were dissolved in excess NaCN in deionized water. The organic solvent used in the liquid membranes was n-heptane, which is a commercially available solvent. All the chemicals were used as received. LIX 79 (N,N′-bis(2-ethylhexyl)guanidine) was kindly donated by Henkel Co. The HFM is manufactured by Hoechst Celanese, Charlotte, NC (Liqui-Cel, 8 × 28 cm 5PCG-259 contactor and 5 PCS-1002 Liqui-Cel laboratory LLE) as specified below. type of module no. of fibers module diam (cm) module length (cm) active interfacial area (m2)

Figure 1. Schematic view of hollow fiber supported liquid membrane run in the recycling mode for the recovery of Au(I) from aqueous cyanide media: 1, hollow fiber contactor; 2, 3, feed and strip pump; 4, 5, feed and strip reservoir, respectively.

5PCG-259 (contractor) 10 000 8 28 1.4

Hollow fiber membrane details are as follows: fiber i.d. (cm) 24.0 × 10-3 fiber o.d. (cm) 30.0 × 10-3 fiber wall 3.0 × 10-3 thickness (cm) fiber length (cm) 15 porosity (%) 30

pore size (µm) polymeric material tortuosity area per unit volume

0.03 polypropylene 3 29.3 (cm2/cm3)

HFSLM Preparation and Methods. Impregnation of the solvents on the polymeric support was carried out by pumping the organic solvents through the fiber bore for 1 h at a slow flow rate. Solvent flowed rapidly through the porous wall of the fibers and was collected in the outer shell. This probably indicates that the impregnation was completed in a few minutes. The module was run in recycling mode, and a schematic of the process is shown in Figure 1. In this mode, feed containing Au(I) in alkaline cyanide media and strip (0.4-1 M NaOH) solutions are recirculated in stirred reservoirs. Samples of 1 cm3 each were taken from the feed and stripping tanks at different times. Membrane permeabilities were determined by monitoring Au(I) concentration by atomic absorption spectrometer (2380 Perkin-Elmer absorption spectrometer) in the feed as a function of time. In the experiments dealing with the

Figure 2. Liquid-liquid extraction plots (a, top) log Dr vs initial concentration of gold in aqueous solution and (b, bottom) log Dr vs LIX79 concentration % (v/v).

separation of Au(I) from base metals in the hollow fiber contactor, the mixture containing Au(I), Cu(I), Fe(II), and Zn(II) was analyzed by ICP (Spectroflame by Spectro Analytical Instruments) to determine each metal concentration. Partition Coefficients of Au(I) and Extraction Equilibrium. Details of the liquid-liquid distribution measurements are essentially the same as those published elsewhere.14 The values of Kex for Au(I) with LIX 79 were found to be (1.58 ( 0.15) × 1011. Parts a and b of Figure 2 depict the log D values as a function of initial concentration of gold in aqueous solution and LIX79/nheptane concentration, respectively. The details of these

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figures will be discussed in Results and Discussion in order to correlate the role of the partition coefficient with the permeability behavior of gold.

(i) for the feed phase circulating through the inside of the fiber (A/Vm)in )

Theoretical Background Permeation Definition. For the recycling mode, both feed solution and stripping solution are recycled, as shown in Figure 1. To model the recycling mode, in 1984 Danesi6a proposed a simple model with a constant permeation coefficient. The difficulties involved in describing a non-steady-state process with variation of the concentration in the axial and radial directions making use of the continuity equation led to the use of the macroscopic mass balance of the permeating solute in a certain volume of fiber in a given time interval.15,16 The model for the transport of Au(I) in a hollow fiber supported liquid membrane system operating in a recycling mode consists of four equations describing (i) the change of the Au(I) concentration in the feed and stripping streams when circulating through the membrane module and (ii) the change of the Au(I) concentration in the feed and stripping tanks, where the aqueous solutions are continuously recirculated, based on the complete mixing hypothesis. Assuming linear concentration gradients and the lack of back-mixing, these equations are formulated as follows:

for the feed solution module mass balance

( )

∂Cm ∂Cm f f A ) -νf ∂t ∂z Vm

m PAu(Cm f - Cs )

(2)

in

Qf m dCTf m ) (Cf,z)L - Cf,z)0 ) dt Vf

(3)

for the stripping solution module mass balance

( )

∂Cm ∂Cm s s A ) -νs + ∂t ∂z Vm

m PAu(Cm f - Cs )

(4)

out

tank mass balance dCts Qs m m ) (Cs,z)L - Cs,z)0 ) dt Vs

(5)

where PAu is the overall permeability coefficient (cm/s), C is the solute concentration (g/cm3), L is the fiber length (cm), Q is the flow rate (cm3/s), ν is the linear velocity (cm/s), and V is the tank volume (cm3). The subscripts f and s refer to the feed and stripping solutions, respectively. The superscripts m and T refer to the membrane module and phase tank, respectively. A/Vm is the ratio of the area of the volume of mass transfer of the fiber:

2

)

πnfri L

2 ri

(6)

(ii) for the stripping phase circulating along the outside of the fiber (A/Vm)out )

2πnfroL 2

2

π(Rc - nfro )L

)

2ronf 2

Rc - nfro2

(7)

where nf is the number of fibers contained in the membrane module, Rc is the inner radius of the module cell, and ri and ro are the inner and outer radii of the hollow fiber, respectively. The integration of the system of differential eqs 2-5 for concurrent flow can be obtained by numerical methods. When a sodium hydroxide solution is used as the stripping agent, an instantaneous reaction is assumed to occur on the outside of the fiber, leading to T Cm s ) 0 and Cs ) 0. In this case, the solution to eqs 2-5 is simplified to

Vf ln

( ) {

( )}

Cf,t)0 2PAuL ) Qf 1 - exp t Cf νf r i

(8)

Experimental results can thus be fitted to a first-order kinetic law f /CA) ) St Vf ln(CA,t)0

(9)

where S is the factor dependent on the geometry of the fibers and the module, the linear velocity of the fluids, and the overall permeability of the system. The overall permeability coefficient can easily be obtained from the experimental value of the slope S as follows:

PAu )

tank mass balance

2πnfriL

[(

νfri S ln 1 2L Qf

)]

(10)

for a system run in a recycling mode. The design of the hollow fiber supported liquid membrane modules for the separation-concentration of gold using overall permeability coefficient PAu centers on three mass-transfer resistances. One of them occurs in the liquid flowing through the hollow fiber lumen. The second corresponds to the gold-complex diffusion across the liquid membrane immobilized on the porous wall of the fiber. The third resistance is due to the aqueous interface created on the outside of the fiber. The reciprocal of the overall permeability coefficient is given by

ri 1 ri 1 1 1 ) + + PAu ki rlm Pm ro ko

(11)

where rlm is the hollow fiber log mean radius and ki and ko are the interfacial coefficients corresponding to the inner and outer aqueous boundary layers. Pm is the membrane permeability, which is related to the partition coefficient of gold (Dr) with LIX7914 by

Pm ) Drkm ) Kex[H+][R]orgkm where the partition coefficient Dr is defined by

(12)

Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 149

Dr ) [Au(CN)2R]org/[Au(CN)2-]

(13)

Inserting eq 16 in eq 15 gives PAu:

ri 1 ri 1 1 1 + ) + + PAu ki rlm k K [H ][R ] ro ko m ex org

(14)

When the reaction is instantaneous on the stripping side, the contribution of the outer aqueous phase is removed from eq 14 and PAu is determined from

ri 1 1 1 ) + PAu ki rlm k K [H+][R ] m ex org

(15)

Membrane Diffusion. The effective diffusion coefficients (Deff) of Au(I) extractant complexes through the organic membrane phase were determined through the model. An effective diffusion coefficient (Deff) for the solute in the immobilized organic liquid membrane can be defined as follows:

Deff ) kmtmτ

Figure 3. Concentration courses obtained from the feasibility study in (O) the feed tank and (9) stripping tank.

(16)

Results and Discussion HFSLM System. Feasibility Studies. Several authors have undertaken studies to improve the stability of HFSLM systems either by facilitating a continuous impregnation method17 or by selecting aliphatic diluent18 or even mixing organic extractant with feed.19 In view of this, to obtain stable HFSLM, an aliphatic diluent, n-heptane, was selected as diluent. In previous studies performed by Sirkar et al.,4 n-heptane was reported as being a promising diluent. Furthermore, in a recent review, improved configurations of liquid membranes are described by Sastre et al.;9 they focused on the research done toward stabilizing hollow fiber SLMs and other LM configurations. The initial approaches to the study of SLM stability are experimental works20,21 in which the variation in the overall permeability coefficient is used as a measurement of the degree of liquid membrane instability. The reasons for the possible causes of SLM instability have been described elsewhere.9,18 Working in a recycling experimental setup, the feed solution consisted of 2000 cm3 of an aqueous solution with an initial Au(I) concentration of 10 mg/L circulating at a flow rate of 12 L/h; 350 mL of 0.4 M NaOH was used as stripping solution at the flow rate of 12 L/h. Experimental results expressed as the Au(I) concentration in the feed and stripping tanks against time obtained in the HF module (details mentioned in experimental procedure) are given in Figure 3. Au(I) concentration was reduced to 0.1 mg/L in the feed solution, whereas it was concentrated up to 60 mg/L in stripping solution. As a continuation of this experiment, the Au(I) concentration in the stripping solution could be achieved up to 400 mg/L when feed phase was continuously replaced with fresh solution, and the same stripping solution (without replacement) was maintained throughout these experiments. The concentration factor for Au(I) defined as the ratio of the final concentration of Au(I) in the stripping solution to the initial concentration in the feed solution was found to be around 40. The feasibility of recovering Au(I) with the hollow fiber module using LIX79 in n-heptane as liquid membrane was thus proved.

Figure 4. Stability analysis of the HFSLM system investigated in n-heptane. The experiment was run for 200 h without renewal of the liquid membrane.

A similar experiment was run for 200 h in order to check the applicability of the process at the long times usually required for industrial operations. The Celgard X-30 fiber was impregnated with LIX79/n-heptane, and the experiment was carried out without any further regeneration of the liquid membrane. The feed phase (2000 cm3) consisted of Au(I) solution with an initial concentration of 10 mg/L in alkaline cyanide media (pH ) 10.3), which was periodically renewed. A sodium hydroxide aqueous solution (0.4 M) was used as stripping reagent. The flow rates of both feed and stripping solution were held constant from the previous viability experiment. Figure 4 gives the experimental results, showing that the transport rate of Au(I) is held constant during the time period considered. Influence of the Concentration of the Aqueous Phases and Hydrodynamics. For the better understanding of the permeability behavior of Au(I), one should have the knowledge of the extraction and stripping reactions and their associated components. The Au(I) ions in alkaline cyanide media (present as Au(CN)2-) form a complex (ion-pair type) with the extractant LIX 79 (N,N′-bis(2-ethylhexyl)guanidine, RH), expressed as

Rorg + H+aq + (Au(CN)2-)aq S [HAu(CN)2R]o

Kex (17)

and stripping of Au(I) from the loaded LIX79 phase is shown as

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[HAu(CN)2R]o + NaOHs S (Au(CN)2-)s + Ro + H2O (18) To study the influence of the initial concentration of Au(I) in the feed solution and sodium hydroxide concentration in the stripping solution on the overall mass transfer rate, two series of experiments were performed. Recycling the feed stream at a linear velocity νf ) 0.74 cm/s, the stripping solution was recycled at νs ) 0.20 cm/s. The initial Au(I) concentration in the feed solution (low metal concentration range) is in the 10-50 mg/L range. To check the stripping solution concentration effect on mass transfer, experiments were carried out using sodium hydroxide with initial concentrations in the 0.3-1 M range. The experimental results were fitted to eq 9. Figures 5 and 6 plot VA ln(Co/C) against time, showing first order for different initial Au(I) and NaOH concentrations, respectively. Working with an initial Au(I) concentration in the 10 e CAu(I) e 50 mg/L range, the experimental courses are independent of metal as well as NaOH concentration (0.3-1 M) range. As seen from Figure 2, Dr values fall in the same range for the gold concentration, namely, 10-50 mg/L. Hence, permeability coefficient was independent of initial metal concentration and could be evaluated by eq 10 which is independent of metal concentration. However, a lower concentration of NaOH (0.1-0.2 M) affected recovery of gold in the receiving phase, and hence a higher concentration was used. The overall permeability coefficient (PAu) determined from the slope in Figure 5 was 4.2 × 10-5 cm/s and from the slope in Figure 6, 4.7 × 10-5 cm/s. The small increase in a PAu value obtained from Figure 6 is due to the slightly high mass transfer caused in the presence of 1 M NaOH, which minutely increases the overall slope. As seen from eq 18, more concentration of NaOH will result in effective removal of gold from the membrane phase. Probably, more concentration of NaOH would shift the reaction to the right, which ultimately increases the gold stripping in the receiving phase. Table 1 presents PAu values at different initial metal concentrations in the feed using 8% LIX79 in n-heptane at a linear flow velocity of 0.74 cm/s. The lowest permeability coefficients were obtained for the highest concentration of gold (243 mg/L) in the feed. This effect is further discussed in detail in the modeling section, while diffusion coefficients are evaluated in different chemical conditions. As indicated in Table 1, permeability coefficient value (PAu) at a 10 mg/L concentration of metal was 4.2 × 10-5 cm/s, which further plummeted to 0.8 × 10-5 cm/s when the metal concentration increased to 243 mg/L. As seen from eq 12, permeability is correlated with partition coefficient. Figure 2 indicated that the log D value decreased with an increase in the metal concentration which proved to be the same behavior in permeability experiments. Since we know that the reciprocal of km is the mass transfer resistance (Pm ) [Dr/(1/km)]), hence this clearly shows that membrane permeability is entirely controlled by membrane diffusion. Furthermore, the effect of pH was an important parameter to be considered in order to optimize the working pH range. Figure 7 presents experimental courses (ln(C/Co) vs time) at different initial feed pH, which shows that metal permeation increased with the decrease in pH, as expected from eq 5. The recovery of gold was drastically affected between pH 11 and pH 12, as seen in Figure 7. These results are similar to those

Figure 5. VA ln(Co/C) against elapsed time from the study of the influence of the initial Au(I) concentration. The S factor is the slope of the regression line.

Figure 6. VA ln(Co/C) against elapsed time from the study of the influence of the initial stripping phase NaOH concentration. Table 1. Experimental Values of the Overall Permeability Coefficient (PAu) for the Recycling System Using Different Initial Metal Concentrations in the Feed (Feed pH, 10.3; Receiving Phase, 0.4 M NaOH; Feed Linear Flow Velocity, 0.74 cm/s) Au(I) concn (mg/L)

factor S (cm3/min)

r2

PAu (cm/s)

10 166 243

26.28 6.88 5.08

0.9935 0.9869 0.9740

4.2 × 10-5 1.0 × 10-5 0.8 × 10-5

Figure 7. Influence of the initial pH on the permeability of Au(I) in the feed phase as a function of time.

previously observed with the flat-sheet supported liquid membrane studies of Au(I) from alkaline cyanide media (with the same feed composition) using LIX79/cumene as liquid membrane, which reinforces the previous assumption that the driving force for Au(I) transport is the chemical potential gradient of complexing species on both sides of the liquid membrane. The aqueous feed with pH 10.3 was deliberately selected, considering the real hydrometallurgical solutions, which have a pH between 10.0 and 10.5. In our HFSLM system, 0.4 M of NaOH gave better 95% permeation of Au(I) when NaCN concentration was almost negligible in the feed. In the

Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 151 Table 2. Experimental Values of Overall Permeability (PAu) for the HFSLM System Run in a Recycling System Using Different Concentrations of LIX 79 in n-Heptane (Feed pH, 10.3; Receiving Phase, 0.4 M NaOH; Feed Linear Flow Velocity, 0.74 cm/s; Au(I) Concentration in Feed, 10 mg/L)

Figure 8. Experimental courses of Au(I) concentration in the feed tank: effect of the initial concentration of NaCN on the feed phase, Au(I) ) 10 mg/L; receiving phase, 1.0 M NaOH, pH of aqueous feed ) 10.3.

Figure 9. Effect of feed linear flow velocity on Au(I) permeability, Au(I) ) 10 mg/L: receiving phase, 0.4 M NaOH; pH of aqueous feed, 10.3; LIX79/n-heptane, 8%.

presence of a high concentration of NaCN in the feed, no efficient stripping was achieved with a low concentration of NaOH (0.4 M NaOH). Greater concentration of NaOH was required for Au(I) removal when the feed contains a higher concentration of NaCN (1000 mg/L). Therefore, in studies performed to separate and concentrate Au(I) in the presence of 0-1000 mg/L NaCN and other metal cyanides, 1 M NaOH was used as the stripping solution. Experimental courses of dimensionless Au(I) concentration with varying NaCN concentration in the feed against time are shown in Figure 8. The same behavior was also observed when Virnig et al.13 performed liquid-liquid extraction studies with gold mine solutions containing Au(I) (1.3 mg/L) from alkaline cyanide media (5000 mg/L NaCN) using 15% (v/v) LIX 79 in Exxon Aromatic 150 with modifier 50 g/L tridecanol. They suggested that a minimum NaOH level of 28-30 g/L was required in the strip solution to maintain acceptable stripping performance with feed mine solution in the presence of other metal cyanides salts such as Ag(I), Zn(II), Ni(II),Cu(I), Fe(II), and Se(IV). As shown in Figure 8, Au(I) removal from the feed was not affected significantly in the presence of NaCN except in the case when the NaCN concentration was 1000 mg/ L. This is probably due to a high level of free cyanide which competes for reagent with aurocyanide as suggested by Virnig et al.13 To minimize this effect, a higher concentration of LIX79, i.e., 12% (v/v), was tested and recovery of Au(I) was found to be closer to the data indicated in Figure 8. Figure 9 presents the effect of different feed linear flow velocity for testing 10 mg/L concentrations of Au(I) using 8% of LIX79/n-heptane. PAu values increased with increasing νf up to 0.98 cm/s and further slightly

LIX 79 concn (% (v/v))

factor S (cm3/min)

r2

PAu (cm/s)

2 4 8 12 16

4.46 18.23 26.28 111.05 159.27

0.9935 0.9882 0.9935 0.9928 0.9874

6.7 × 10-6 2.8 × 10-5 4.2 × 10-5 2.4 × 10-4 4.7 × 10-4

decreased at a linear flow velocity of 1.54 cm/s. As expected, PAu first increased with νf and then becomes independent of it or slightly decreased. The increase of PAu with νf is caused by a decrease of the thickness of the aqueous boundary layer when the νf in the fiber lumen increased. Further, PAu becomes independent of νf when aqueous mass-transfer resistance (1/ki) becomes negligible with respect to membrane mass-transfer resistance (1/km). Even though the permeability is slightly higher at a linear flow velocity of 0.98 cm/s in one batch but a lower flow rate, 12 L/h (ν ) 0.74 cm/s) was deliberately selected in order to maintain HFSLM stability in the long run of experiments, which could be affected at high flow rates. This is probably due to increased pressure drops across the membrane with an increasing flow rate, which results in expulsion of the organic phase from the membrane and ultimately reduces the metal removal in the hollow fiber module. The other reason for not selecting high flow rates was the formation of emulsion at the source/membrane interface due to lateral shear forces.22 This process will indeed reduce the effective mass-transfer rate at the feed/membrane interface, and this decrease will be more noticeable with increasing flow rates. A separate experiment was performed to evaluate the effect on metal removal when feed flowed though shell side and stripping solution through tube side. Gold recovery was not affected significantly in the stripping solution but the overall permeability coefficient was almost one-fourth of that obtained by the flowing feed tube side and stripping solution shell side. The reduction in overall permeability coefficient results due to the linear flow velocity of the feed which decreased from 0.74 to 0.20 cm/s when the solution passed though the shell side. Another reason for the decrease in gold cyanide permeability was probably the insufficient membrane area provided for the stripping reaction in the tube side as compared to the shell side. Hence, the other mode (flowing feed through the tube side) of operation was adopted throughout in order to obtain a high mass transfer. Influence of LIX 79 Concentration and Membrane Composition. The membrane composition influence on the gold separation rate was investigated in order to optimize the LIX 79 concentration. The liquid membrane consisting of 2-16% LIX 79 in the recycling mode scheme was tested. Table 2 summarizes the factor S and the overall permeability coefficient (PAu) for different concentrations of LIX79 at the linear velocity of 0.74 cm/s. Figure 10 is a plot of VA ln(C/Co) against time for different LIX79 concentrations from the batch. The permeability was seen to increase with extractant concentration up to 16% (v/v), and the transport rate is therefore limited by diffusion through the aqueous film on the feed side of the membrane in this region. This trend was also indicated by Figure 2, which shows the

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Figure 10. VA ln(Co/C) against elapsed time for different concentrations of LIX79 from the experimental results. The S factors are the slopes of the regression lines (conditions the same as in Figure 9 except LIX79 concentration).

Figure 12. [Au(I)0in] - [Au(I)]in plotted vs time (t) for the higher concentration of gold using 8% LIX79.

shifted to the right. Dr is not any more independent of initial metal concentration, C. The highest value of gold complex concentration is equal to total carrier concentration ([R]org/n and the partition coefficient Dr ) [R]org/ C becomes inversely dependent on C. n is the number of carrier molecules per metal ion in the metal carrier complex. Therefore, for large concentrations of C, when 1/ki , 1/km,6b

P)

Figure 11. Plot of 1/PAu as a function of 1/Kex[H+][R]org (see details in text).

increase in the log Dr value with an increase in the LIX79 concentration. Regression lines were drawn, showing a good match with eq 9. The S factors were calculated from the slopes. Equation 10 allows experimental values of PAu to be evaluated for batch experiments. Evaluation of Diffusional Parameters. To evaluate the aqueous mass-transfer coefficient (ki), the membrane mass transfer coefficient (km), and the diffusivity, eq 15 can be used to calculate these parameters. By plotting 1/P as a function of 1/Kex[H+][R]org, for different extractant concentrations of LIX 79 (at aqueous pH 10.3), and varying pH with constant 8% LIX79 concentration, one should obtain a straight line with slope ri/rlmkm and an ordinate to calculate 1/ki (Figure 11). The values of ki and km calculated from the proposed model are 1.8 × 10-4 cm/s and 2.8 × 10-4 cm/s, respectively. The calculated value of the effective diffusion coefficient (Deff ) τdokm) was Deff ) 2.5 × 10-6 cm2 s-1. This closely coincided with the magnitude of diffusion coefficient values evaluated by Danesi et al.23 in their previous work while studying the Am(III)CMPO-SLM system from aqueous nitrate media using Accurel BS7C (polypropylene) support. Although, the author has not mentioned the tortuosity factor for Accurel BS7C (polypropylene) support but tortuosity values have been estimated according to the following relation (τ ) 1 + Vp/1 - Vp, where Vp ) 1 -  is the volume fraction of the of the polymeric framework).18 The tortuosity was determined to be 3, which is in the same range of the hollow fiber membrane used in the present study. The feed solutions contain metal ions at relatively high concentrations, and reaction (eq 17) is completely

J [R]orgkm ) C nC

(19)

Hence, eq 9 is no longer valid; therefore the following equation can be used for the HFSLM system run in the recycling mode6a

[Au(I)]0in - [Au(I)]in )

[R]kmA t nV

(20)

Figure 12 shows the kind of curve experimentally obtained when the initial metal concentration in the feed was between 9.4 × 10-4 and 1.2 × 10-3 M and the LIX79 in the membrane phase is 0.25 M. These curves are described by eq 20. Furthermore, the slope of the straight line and eq 20 allow us to evaluate the membrane diffusion coefficients of HRAu(CN)2 when all the carrier is bound to the metal. The calculated value is Deff ) 1.7 × 10-7 cm/s, which ties in well with earlier studies reported by Danesi et al.23 for the SLM-Am/SmCMPO system and higher metal concentration using Accurel BS7C (polypropylene, tortuosity value ) 3). Selectivity. Some experiments were conducted to demonstrate the selectivity of the gold in order to examine the effect of several other metal ions generally accompanying Au(CN)2- and their interferences with the overall permeation of Au(I). In cyanide leaching or electroplating solutions, there are hosts of complex cyanoanions that can be found. Among the metal ions examined, Fe(II) (30 mg/L), Cu(I) (30 mg/L), Ni(II) (5 mg/L), and Ag(I) (3 mg/L) were tested in the form of a mixture with Au(I) (10.0 mg/L). The cyano ions which exist with Au(I) were Ag(CN)2-, Cu(CN)42-, Ni(CN)42-, and Fe(CN)64-. Gold selectivity with respect to these anions was determined for 12% LIX79. These conditions were similar to those encountered in hydrometallurgical leach solutions from low-grade ores. Figure 13 presents the results of Au(I) permeability in the presence of base metals such as Cu(II), Ni(II), Ag(I), and Fe(II). The separation factors based on the experimental results obtained for the cyano ions by LIX79 gives the following order of selectivity (Table 3):

Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 153

Conclusions

Figure 13. Separation of Au(I) from Fe(II), Cu(II), Ag(I), and Ni(II): Au(I), 10.0 mg/L; Fe(II), 30 mg/L, Cu(II), 30 mg/L; Ni(II), 10 mg/L; Ag(I), 3 mg/L and in feed at pH 10.3; NaCN concentration in feed, 1000 ppm; extractant concentration, 12% (v/v) LIX79; stripping solution, 1.0 M NaOH. Table 3. Experimental Values of the Overall Permeability (PAu) for Different Cyanide Metal Salts in the Feed for HFSLM in Recycling Mode Using LIX79 in n-Heptane (Feed pH, 10.3; Receiving Phase, 1 M NaOH; Feed Linear Flow Velocity, 0.74 cm/s; LIX79 in n-Heptane, 12% (v/v); NaCN Concentration in Feed, 1000 mg/L) metal ions metal concn (mg/L) PAu (cm/s) factor S (cm3/min) Au(I) Fe(II) Ni(II) Cu(I) Ag(I)

10.0 30 5 30 3

8.8 × 10-5 4.3 × 10-8 5.2 × 10-6 2.4 × 10-6 6.7 × 10-6

2046.5 16.9 36.7 13.1

Acknowledgment This work was supported by the CICYT (Grant QUI99-749) and CIRIT (Grant SGR-98-0082). A.K. acknowledges financial support from the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain, in the form of a Visiting Scientist fellowship. The authors would also like to thank Jordy and Belen for performing the experiments and Henkel Corp. for supplying LIX79. Nomenclature

Au(CN)2 > Ag(CN)2 > Ni(CN)42-, >Cu(CN)43-, >Fe(CN)64Under these circumstances, significant discrimination between the extraction of Au(CN)2- and other cyano ions can be achieved, as shown in Figure 13. For this study, the same chemical conditions (pH ) 10.3, flow rate of feed ) 0.74 cm/s, and stripping phase NaOH ) 1 M, NaCN ) 1000 ppm) as those for the Au(I) were chosen, using 12% LIX79 as the organic extractant. Table 3 lists the PAu values of different metal cyanide salts and separation factors. The PAu value of gold using 12% LIX79 in the presence of other metal cyanide salts was reduced from 2.4 × 10-4 to 8.8 × 10-5 cm/s, which is probably due to multiion competition. Interestingly, the initial 60% of Au(I) separation was achieved in the first 50 min of HFSLM operation when other metal cyanide salts were permeated under 10%. Hence, clean permeation of Au(I) is observed in this region. The separation factor of each base metal is defined as:

SFAu ) PAu/PM

The hollow fiber supported liquid membrane technique was found to be a promising for the simultaneous separation and concentration of Au(I) from alkaline cyanide media in the presence of other metal cyanides such as Ag(CN)2-, Cu(CN)43-, Ni(CN)42-, and Fe(CN)64using LIX79 in n-heptane. The use of LIX79/n-heptane immobilized on polypropylene hollow fiber resulted in systems whose stabilities were experimentally tested for long operation times (up to 200 h) for efficient Au(I) removal. The use of stripping solution containing NaOH provided efficient and fast stripping of Au(I) in receiving (product) solutions. A good selectivity was found, and the separation factors based on the experimental results obtained for the cyanoions by 12% (v/v) LIX79 give the following order of selectivity: Au(CN)2 f Ag(CN)2 f Ni(CN)42-, >Cu(CN)43-, >Fe(CN)64-.

(21)

where M is the metal ion tested for selectivity. The permeability of iron was found to be nil, and the separation factors of gold with respect to Fe(II) and Cu(II) were 2046.5 and 36.7, respectively. The most important factor which controls the extraction order of cyano anions appears to be charge. For anions of the same type, particularly Au(CN)2- and Ag(CN)2-, the larger Au(CN)2- anion is extracted in preference to Ag(CN)2-, and other highly charged cyano anions were not amenable to extraction by LIX79 under similar experimental conditions. Similar results were found in the extraction of cyano anions by quaternary amines.24

C ) metal concentration (g/cm3) d ) diameter of one fiber (cm) da ) thickness of the aqueous feed boundary layer (cm) Dr ) partition coefficient of gold di and do ) inner and outer fiber diameters, respectively tm ) thickness of the fiber membrane (cm) Deff ) effective membrane diffusion coefficient of the goldcontaining species ki ) aqueous mass transfer coefficient km ) membrane mass transfer coefficient ko ) mass transfer coefficient in the strip L ) fiber length (cm) nf ) number of fibers Q ) flow rate (cm3/s) ri and ro ) inner and outer hollow fiber radii (cm) SF ) separation factor PAu ) overall permeability coefficient (cm/s) V ) tank volume (cm3) Vm ) volume of hollow fibers (cm3) Subscripts f and s ) feed and stripping solutions, respectively in ) inside the fiber out ) along the outside of the fiber i ) inner radii o ) outer radii Superscripts m and T ) membrane module and phase tank ° ) concentration at time zero Greek Letters τ ) tortuosity of the membrane νf and νs ) velocity of liquid inside fiber and shell side (cm/ s)

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Received for review April 12, 1999 Revised manuscript received August 5, 1999 Accepted August 19, 1999 IE990267A